Earth With Two Moons: Maybe Not Just Science Fiction

Download PDF

How’d the moon get to be floating around the Earth? It’s a question a lot of people have asked over the eons.  It used to be that question was answered with myth and legend, giving the moon – like the earth – divine origins. I like to answer origin-questions that same way in some parts of my science fiction, and often have a piece of folklore that fills in the blanks on such topics. In fact, I have a folktale called “Rulandor’s End,” that explains how the little, fast-moving moon in Casca’s1 night sky came to be.  (I’ll be uploading that story very soon, along with some other content in my Sa’adani Empire section here, and will be updating this post as soon as that’s online.)

Scientists usually have a different take on such things, however.

For quite a while, our researchers have had a theory that the Earth’s moon was created by a giant impact. They posit that a Mars-sized planet smashed against the Earth. The impact ejected a lot of mass into space; that mass then swirled around and eventually coalesced into the moon.

But there’s an odd thing about our moon. The side that faces us is relatively smooth and featureless, but the far side is quite rough and mountainous. This is not in keeping with the form you’d expect for a globe where gravitational attraction caused the mass to come together uniformly around a center.

A new hypothesis has emerged to explain this asymmetry. Researchers at the University of California Santa Cruz believe it is possible that we once had two moons orbiting the earth, probably both created by the giant impact event.  Eventually, they collided – a slow collision, not strong enough to obliterate both, but grinding one into the other and deforming the planetoid that survived the collision.  A collision of this sort would not create craters or melting, but would deposit a new layer of thick crust on the impacted area.

“Our model works well with models of the moon-forming giant impact, which predict there should be massive debris left in orbit about the Earth, besides the moon itself,” said Professor Erik Asphaug, lead researcher on this work2.

His colleague Professor Francis Nimmo, one of the authors of the “tidal forces” theory, said “It agrees with what is known about the dynamical stability of such a system, the timing of the cooling of the moon, and the ages of lunar rocks…The fact that the near side of the moon looks so different to the far side has been a puzzle since the dawn of the space age, perhaps second only to the origin of the moon itself.”

The event might have looked something like this:

Two moons colliding.
Artist’s rendering of smaller moon colliding with our moon about 4 billion years ago. Credit: Martin Jutzi and Eric Asphaug, UCSC (

That is interesting enough as a theory, but you know where my science fiction imagination runs with that? Right here:

How would that have looked, seen from the Earth? What tidal effects would it have created, or how devastating were the impacts from debris cast through our atmosphere?

And even more spectacular and down-right terrifying: how would that look if you lived on the moon involved?

Science fiction has all manner of stories that take place on moons.  From Heinlein’s classic The Moon is a Harsh Mistress to Piers Anthony’s intriguing Jupiter moon settings in the Bio of a Space Tyrant cycle, to Yavin 4, the rebel base in Star Wars IV from which the Death Star-destroying attack was launched – we do like our moons in this genre and we’re not afraid to live on ’em!

So what if you were living on one when another moon comes a-calling? You can run, but you can’t hide. Talk about an end-of-the-world scenario, writ small but no less deadly for any lifeforms in the way of it.


Feels like something a short story should be written about.

Just sayin’…

Loving that new hypothesis in a perverse science-geek-cum-story-teller kinda way.


1. Casca is a world in old Sa’adani space in my science fiction settings where I’ve been doing considerable development and world building. It has not yet made an appearance in my published works, but probably will in the future.

2. Lovett, Richard. “Early Earth may have had two moons.” Nature News, Aug 3, 2011.

 Subscribe in a reader

Did you enjoy this post? Why not leave a comment below and continue the conversation, or subscribe to my feed and get articles like this delivered automatically to your feed reader.


No comments yet.

Leave a comment